Getting non-UI objects to respond to WPF command bindings

Question

I have a ViewModel class which i want to respond to the built in Refresh command whic is fired from a button but i'm not sure how to declare the CommandTarget.

Briefly, my code is as below

The ViewModel constructor and CanExecute and Executed event handlers -

    public ViewModel()
    {
        CommandBinding binding = new CommandBinding(NavigationCommands.Refresh, CommandHandler);
        binding.CanExecute += new CanExecuteRoutedEventHandler(binding_CanExecute);
        binding.Executed += new ExecutedRoutedEventHandler(binding_Executed);
        CommandManager.RegisterClassCommandBinding(typeof(ViewModel), binding);
    }
    void binding_Executed(object sender, ExecutedRoutedEventArgs e)
    {
        Debug.Print("Refreshing...");
    }

    void binding_CanExecute(object sender, CanExecuteRoutedEventArgs e)
    {
        e.CanExecute = true;
    }

The markup is -

<Button Command="Refresh">refresh</Button>

Now, I've tried setting the CommandTarget on this button to {Binding Source={StaticResource ViewModel}} but i get a runtime saying Cannot convert the value in attribute 'CommandTarget' to object of type 'System.Windows.IInputElement'.

I'm new to commands so it's entirely possible I'm all kinds of wrong here. Anyhelp would be appreciated.

Answer 1

RoutedCommands and MVVM do not mix. RoutedCommands are tied to the visual tree and to rely on WPF's CommandBindings collection. You should implement your own ICommand classes that work with the MVVM pattern. Take a look at Prism's implementations for starters.

In my own MVVM projects, I have a couple of command implementations:

• DelegateCommand. Calls provided delegates to determine whether the command can execute, and to execute the command.
• ActiveAwareCommand. Works in conjunction with an interface (IActiveAware) and sends command executions to the currently active item. Multiple active aware implementations register themselves with the command, and the command automatically routes CanExecute / Execute calls to the currently active item.

HTH, Kent